Trigun, Ansatsu Kyoushitsu Recommendations
Both series revolve around a surprisingly nice living natural disaster, both of whom have enormous bounties on their heads, and take a pacifist approach despite how easily they could kill anyone and everyone who stands in their way.
Both characters caused widespread disaster in their backstory for reasons that are kept hidden. Their motivation revolves around a woman in their backstory whose significance is kept vague.
They also have a brother with similar abilities, who they are enemies with.
Koro-Sensei and Triguns main character Vash the Stampede shares lots of similarities. Both have a bounty on their heads but acts carefree. They are both feard, Koro-Sensei for blowing up the moon and Vach for blowing up cities. However, they are both skilled and are good and strong fighters. Both of them have hidden special abilities and both are fast runners (of course is Koro-Sensei faster). Their backgrounds have also many similarities, but they remains unknown or vague, but is reviled later in (Koro-Sensei’s is reviled in season 2).
Oh, and both have a sweet-tooth!